So.... where can I get one of those free medium-format projectors you're basing your pricing on?

My projectors have already paid for themselves......
Well for used medium format slide projectors I recommend looking at online auction sources in Germany. Because of the popularity of slide projection in Germany you have better chances to find one on the German market.
Or contact some distributors who sell also used gear, like

Slide projectors like a Götschmann, the Leica projectors or the Rollei 66 dual P are very robust build gear. They last more than a lifetime, even our children can use them when we pass away.
They are a "once in a lifetime" purchase.
Therefore even a new Rollei or Götschmann is very cheap if you consider
- their lifespan
- the outstanding picture quality you get
- the much much higher costs of digital projection.

Originally Posted by polyglot

I'd project slides if I could but I shoot 6x7, cannot afford a Goetschmann and haven't seen a good price on something secondhand in Australia. I have vague plans to build one from some enlarger condensers, old RB/RZ lens and a HID lamp but haven't really got the time.

For 6x7 the best option are indeed the Götschmann projectors. But it is almost impossible to find one used. Fortunately they are still produced new (but of course nobody knows for how long...).
They are so hard to find because
- those who have one keep it and protect it like a treasure
- only about 2700 samples have been built so far; it is like Rolls-Royce.....;-)

Sometimes the Pro Cabin 6x7 can be found used. But not as good as Götschmann.

For 4,5x6 and 6x6 it is a bit easier to find good projectors used. Sometimes the
- Hasselblad PCP
- Kindermann 66 T (for this projector the 2,8/150 lens is best; avoid the cheaper 3,0/150)
- Rollei P11; Rollei P66, Rolleivision 66, Rolleivision 66 AV
- Rollei 66 dual P
is offered used.
Fortunately the Rollei dual 66 P is still produced new. It is an excellent choice, the best projector in its class.
And very versatile, because it is a multi-format projector. It can be used for
- 35mm slides (24x36mm)
- 4x4 slides (so called "Super-Slides")
- 4,5x6
- 6x6.